When you use one of the methods to query for posts (query_posts(), get_posts() or WP_Query object) arguments you provide are processed and turned into SQL query. This happens in WP_Query->&get_posts() method.
Since arguments are not omnipotent there are a lot of hooks in there that allow to modify or override parts of resulting SQL query.
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posts_joinis part of query that handles SQL JOINs – adding additional tables to the mix, for example tables related to taxonomies when they are needed. -
posts_fieldsseem to control which database fields will be returned in query, it seems to default to all fields frompoststable.
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